The present invention relates generally to the field of resonating devices, and more particularly, is directed to a piezoelectric resonator and piezoelectric filter which are manufactured using thin film technology in order to obtain a high resonant frequency.
In recent years, great progress has been made in the areas of material and process technology for making electronic components such as integrated circuits. As a result, the use of electronic circuitry in manufactured goods has increased over the years and continues to increase at a rapid rate. Unfortunately, advances in passive component parts, such as resonators and filters, have not kept pace. One example of this is the presently unfulfilled need for a compact resonator for communications use in the VHF or UHF bands.
Conventional resonators and filters employing a vibrating piezoelectric substrate, such as crystal, are widely known in the art. Vibrating frequency is a function of substrate thickness. However, the thickness of such a substrate can only be reduced to about several tens of microns in order to maintain sufficient mechanical strength of the substrate. The thickness of the substrate is also limited by present manufacturing and process technology. Thus the upper resonant frequency limit of a conventional resonator is typically only about several tens of MHz. With a piezoelectric substrate, a higher resonate frequency can usually only be obtained by employing a higher-mode vibration. However, the higher the mode, the smaller the electromechanical coupling coefficient becomes, and thus the frequency bandwidth becomes narrower. It is therefore not practical to use a conventional piezoelectric substrate at a higher mode.
A resonator employing a so-called piezoelectric thin-film has recently been proposed for the purpose of making a compact VHF/UHF-band resonator which operates in the fundamental mode or at a relatively low frequency overtone. An example of such a piezoelectric thin-film resonator is disclosed in an article entitled "ZnO/SiO.sub.2 -Diaphragm Composite Resonator on a Silicon Wafer" published in Electronics Letter, July 9, 1981, Vol. 17, No. 14, pp. 507-509. FIGS. 1 and 2 of the present drawings illustrate the structure of this type of piezoelectric thin-film resonator. FIGS. 1 and 2 are perspective and cross-sectional views, respectively, of a piezoelectric thin-film resonator which employs zinc oxide (ZnO) as the piezoelectric film. This type of resonator is manufactured as follows. Film layers 2 and 3 are made of SiO.sub.2 and are formed on opposing sides of a silicon substrate 1. A portion of film layer 3 is removed from the substrate. Using film layer 2 and the remaining portion of Cfilm layer 3 as a mask, substrate 1 is anisotropically (crystal orientation dependent) etched to form a concave portion 4. Concave portion 4 is etched completely through substrate 1 to film layer 2 as shown in FIG. 2. A first electrode 5 is then formed on film layer 2 with a ZnO piezoelectric thin-film layer 6 formed over film layer 2 partially covering first electrode 5. A second electrode 7 is formed on thin-film layer 6 with at least a part of second electrode 7 over first electrode 5.
When a signal is applied to electrodes 5 and 7, the composite film, consisting of thin-film layer 6 and film layer 2 over concave portion 4, vibrates due to the piezoelectric effect of thin-film layer 6 and thus operates as a resonator.
This type of piezoelectric thin-film resonator has the following features:
(1) It can operate over a frequency range from 100 MHz to several GHz in the fundamental and lower overtone mode because its vibrating portion can be made extremely thin.
(2) It has a high electromechanical coupling coefficient and, thus can be designed to have a wide frequency bandwidth.
(3) It can be designed to have a zero temperature coefficient; i.e., designed such that the SiO.sub.2 film layers have a resonant frequency temperature coefficient oppoiste to the resonant frequency temperature coefficient of the piezoelectric film layer.
(4) It can be designed to be very compact in size.
(5) It can be readily assembled in an integrated circuit because the manufacturing process for making the resonator is compatible with the manufacturing process used to make common integrated circuits.
Though piezoelectric thin-film resonators having a concave portion in the silicon substrate have the above-described advantages, they also have a number of deficiencies. When the exposed surface of a silicon substrate is etched in PED liquid [pyrocatechine (pyrocatecol) C.sub.6 H.sub.4 (OH).sub.2, ethylene diamine NH.sub.2 (CH.sub.2).sub.2 NH.sub.2, and water, H.sub.2 O], a pyramid shaped void 4 is formed as shown in FIG. 2. This is because the liquid is highly anisotropic with respect to the rate of etching i.e., the rate of etching is higher along the (100) orientation of the substrate than along the (111) orientation of the substrate. Since the rate of etching along the (100) orientation of a silicon substrate can be as low as 50 microns per hour, it would take eight hours to etch a common silicon substrate having a diameter of three inches and a thickness of 400 microns. This is, of course, not suitable for mass production. Moreover, because a concavity is formed in the substrate, the mechanical strength of the substrate is also reduced. As a result, the substrate requires very delicate handling during the manufacturing process. In addition, where other circuit elements are formed on the same substrate, the process of forming the concavity in the substrate may damage these circuit elements.